DESCRIPTION (adapted from investigator's abstract and/or aims): The catabolism of cholesterol to bile acids and the biliary excretion of bile acids and cholesterol by the liver is the major pathway through which cholesterol is eliminated from the body and cholesterol homeostasis maintained. Alterations in the regulation of this pathway are associated with several pathological conditions, of which cholelithiasis and atherosclerosis are the most common. Data from other laboratories have suggested that the activity of 7-alpha-hydroxylase is depressed in patients with cholesterol gallstone disease. Atherosclerosis is associated with the accumulation of cholesterol carrying lipoproteins (e.g. low density lipoprotein (LDL) in plasma, and there are a multitude of interactions of lipids, enzymes, growth factors and transport processes that underlie atherogenesis. The altered production of bile acids in response to dietary cholesterol has been one factor associated with the susceptibility of certain strains of animals to atherosclerosis. The major goal of the proposed research is to define the molecular mechanisms responsible for regulating bile acid synthesis and 7-alpha-hydroxylase. An additional goal is to determine how expression of 7-alpha-hydroxylase determines cellular cholesterol metabolism and what role it may play in cholesterol gallstone disease and atherosclerosis. The proposal lists three specific aims: 1) To use newly developed reagents and experimental models to determine the molecular and genetic mechanisms regulating 7-alpha-hydroxylase in cultured cells. This will be done using a recently derived line of rat hepatoma cells from the applicant's laboratory that express 7-alpha-hydroxylase in a regulatable manner. The functional significance of 7-alpha-hydroxylase will also be examined by expressing it in non-hepatic cells (CHO cells and mouse RAW macrophages) which have been transfected with a plasmid expressing 7-alpha-hydroxylase. 2) To examine the functional significance of 7-alpha-hydroxylase in cultured cells. The effects of 7-alpha-hydroxylase on the expression of oxysterol regulated genes (hydroxy methyl-glutaryl-co-enzyme A (HMG-CoA) reductase and the LDL receptor) will be examined. The effects on cellular cholesterol homeostasis and efflux will also be examined. 3) To examine the significance of 7-alpha-hydroxylase in-vivo. The functional role of 7-alpha-hydroxylase in cholesterol gallstone formation and atherosclerosis will be examined by expressing it in a constitutive manner in C57BL/6 mice. The investigators have shown that this mouse strain requires inhibited expression of 7-alpha-hydroxylase for the susceptibility of the animals to atherosclerosis and cholesterol gallstone disease. A transgenic mouse strain will be developed in which it will be determined if a non-responsible gene encoding rat 7-alpha-hydroxylase can help prevent the susceptibility of C57BL/6 mice to dietary induced hypercholesterolemia. The combined data should provide new understanding of the role that bile acid synthesis plays in regulating cholesterol metabolism in homeostasis. The studies may also determine the role that 7-alpha-hydroxylase plays in cholelithiasis and atherosclerosis and thereby direct methods which can prevent these disorders.